1. Field of the Invention
The present invention relates to an apparatus and method for control of a melt flow pattern within a crucible, and more specifically to controlling a radial temperature gradient or distribution during crystal growth by the Czochralski method.
The well-known Czochralski method of growing crystals involves contacting a seed crystal with a melt of the crystal material contained in a heated crucible. The growth of large, substantially defect-free crystals by this and other methods remains to this day a difficult task, due to complications brought about by temperature gradients, convective eddies within the melt, and continuously changing thermal conditions.
Previous attempts at controlling the crystal growth process to improve the quality of the crystals produced have primarily involved controlling process parameters such as crucible or crystal rotation speeds, crystal pulling rate and total heat supplied to the crucible. The shape and stability of the solidification interface play an important part in achieving substantially defect-free large single crystals, and the shape and stability depend to a large extent on the flow patterns established in the melt. In practice, control of the crystal and crucible rotation rates have been used to attempt to control the flow pattern in the melt. Application of magnetic fields has also been used in attempting to suppress convective eddies within the melt to control the flow pattern, as well. The above approaches share the disadvantage that the controls tend to produce only bulk effects within the melt. Control or manipulation of the crystal and crucible rotation rates without regard to the prevailing thermal boundary conditions, as has been the practice, leads to inconsistent results. Further, the use of magnetic fields does not guarantee a stable flow regime under the solidification interface.
Heaters disposed below the melt in containers from which single crystals are pulled have heretofore been disclosed, however, these heaters have been designed without the ability to assist in controlling the melt flow pattern in the melt container by selectively controlling and adjusting the thermal gradient across the bottom of the vessel. U.S. Pat. No. 4,654,110, issued to Morrison, discloses a shallow angle sheet crystal puller wherein heating rods disposed in the bottom of the melt-containing vessel have gradually increasing (or decreasing) heating capacity to provide a predetermined temperature gradient in the melt, however, there appears to be no means provided by which variation from the single predetermined gradient could be accomplished.
U.S. Pat. No. 4,659,421, issued to Jewett, discloses a shallow pan-type melt container having more than one heater disposed at the bottom of the container. However, these heaters are designed and used in a manner to minimize the effect on melt flow within the melt container. This Jewett patent discusses in detail the recognition of problems associated with melt flow in the Czochralski crystal pulling process. The invention is directed to a crystal pulling process which is quite different from the Czochralski method in that neither the crystal nor the melt container is rotated during the pulling process, and melt flow within the container is restricted by baffles, and is desirably kept to a minimum. The only desired melt flow using the apparatus and process in this patent is from below the feedstock area to below the crystal pulling area, and the melt flow is controlled either by the pumping action of the feed rod into the melt or by argon gas pressure in the region surrounding the feed rod. No suggestion is made in this patent to selectively control the thermal gradient across the bottom of a crucible to control the melt flow pattern to assist in controlling the shape of the solidification interface.
It is therefore a principal object of the present invention to provide an apparatus which is suitable for controlling the melt flow pattern within a crucible to establish and preserve a desired solidification interface shape.
It is a further principal object of the present invention to provide a method for establishing and maintaining a desired solidification interface shape in a melt by controlling the melt flow pattern within a crucible.
It is an additional object of the present invention to provide heating means disposed at the base of a crucible, and heating means having independently controllable heating elements and being adapted to be moved vertically in conjunction with the crucible to maintain a substantially constant distance between the heating means and the crucible.
It is an additional object of the present invention to provide a method for establishing a stable solidification interface shape by controlling a temperature gradient at a lower portion of the melt whereby a predetermined desired melt flow pattern is created and maintained.